Protein kinases specific for tyrosine have been associated with the control of cell growth. A rabbit antiserum and a mouse monoclonal antibody both specific for phosphotyrosine were developed to identify and purify phosphotyrosine-containing proteins. Using these antibodies we are presently studying phosphorylation of cellular proteins in response to epidermal growth factor. A number of drugs inhibit the phosphorylation response. We will extend these studies to include other growth factors and biologically active substances. Also, we want to examine phosphotyrosine-proteins in a variety of human tumors.